This invention relates to a reactor system and process for the hydrotreating of a heavy feedstock, particularly a residuum, in order to lower the amount of contaminants, especially metals, carbon residue, and sulfur. An upflow fixed bed reactor is described as containing a layered catalyst bed in which the catalyst in the different layers has different hydrogenation activities designed to selectively distribute the removal of the contaminants across the entire catalyst bed to prevent plugging and to increase the life of the catalyst.
Hydrotreating is a well known method for removing contaminants and upgrading heavy feedstocks prior to further processing. The term xe2x80x9chydrotreatingxe2x80x9d will be used in this disclosure to denote a process for removing contaminants, especially metals, carbon residue, nitrogen, and sulfur from heavy feedstocks under supra-atmospheric pressure and at elevated temperatures in the presence of hydrogen and a catalyst. As used herein the term xe2x80x9cheavy feedstockxe2x80x9d refers to a hydrocarbon high in asphaltenes that is derived from a reduced crude oil, petroleum residuum, tar sands bitumen, shale oil, liquified coal, or reclaimed oil. Heavy feedstocks typically contain contaminants, such as carbon residue, sulfur, and metals, which are known to deactivate the catalysts used to upgrade the heavy feedstocks to more valuable products such as transportation fuels and lubricating oils. Hydrotreating operations also typically remove nitrogen from the heavy feedstocks along with the sulfur. Even the production of lower value products such as fuel oils usually requires that the heavy feedstock undergo some upgrading to remove contaminants, especially sulfur, prior to sale in order to reduce air pollution.
Various designs of hydrotreating reactors have been described in the literature for treating heavy feedstocks. Commercial designs may utilize a moving bed of catalyst, such as described in U.S. Pat. No. 5,076,908, or an ebullating catalyst bed, such as described in U.S. Pat. Nos. 4,571,326 and 4,744,887. Downflow fixed bed hydrotreating reactors are the most widely used commercially. They may be distinguished from moving bed reactors in that fresh catalyst cannot be added to the bed and spent catalyst in the bed cannot be removed during operation. In moving bed reactors the flow of feedstock and hydrogen is preferably upward. The catalyst moves downward and is removed from the bottom of the bed as spent catalyst while fresh catalyst is added at the top of the bed. In an ebullating bed the upward flow of feedstock and hydrogen is sufficient to suspend the catalyst and create random movement of the catalyst particles. During operation the volume of an ebullating bed will expand, usually by at least 20%, as compared to the volume of catalyst in the reactor when there is no flow of hydrogen and feedstock through the bed. By contrast, there is little or no expansion in an upflow fixed bed such as described in this disclosure during operation. In fact, the volume of the catalyst bed may actually decrease slightly during operation due to a settling of the catalyst particles. It should be understood that since the reactor walls are rigid the expansion of the catalyst bed will take place only along the vertical axis of the bed. Thus when referring to bed expansion in this disclosure, the increase in height of the bed or depth of the bed in the reactor is an appropriate measure of bed expansion and is directly related to volume.
Usually, the contaminants are removed by contacting the heavy feedstock with a catalyst in the presence of hydrogen at an elevated pressure and temperature. Typically, the catalyst will be an active catalyst, i.e., a catalyst with hydrogenation activity. Contaminating metals, such as nickel and vanadium, usually will be readily removed under hydrotreating conditions and will plate out on the surface and in the pores of the catalyst. The deposition of metals on the catalyst will result in a rapid loss of hydrogenation activity. However, hydrogenation activity is necessary for the removal of other contaminants, such as carbon residue, nitrogen, and sulfur, from the feedstock.
Catalysts used to carry out the removal of metals, carbon residue, and sulfur from heavy feedstocks, referred to generally as hydrotreating catalysts, typically consist of a porous refractory support, usually of alumina, silica, or silica/alumina, that may be impregnated with a metal or metals, such as for example, Group VIB metals (especially molybdenum and tungsten) and Group VIII metals (especially cobalt and nickel) from the Periodic Table, to enhance their activity. Of primary concern with the present invention are those hydrotreating catalysts having demetallation, desulfurization, denitrification, and carbon residue removal activity.
The pore structure of the hydrotreating catalyst is known to affect the desulfurization, denitrification, and carbon residue removal activity of the catalyst as well as how rapidly the catalyst is deactivated by metal contaminants. In general, catalysts having relatively large pores are preferred for removing metal contaminants. For example, catalysts having macropores, that is, pores having diameters of 1000 Angstrom Units or greater, are taught as useful in removing contaminating metals from heavy feedstocks by U.S. Pat. No. 5,215,955. However, for the removal of sulfur, nitrogen, and carbon residue a smaller pore size is usually advantageous, as for example, a catalyst such as described in U.S. Pat. No. 5,177,074 in which at least 70% of its pore volume consists of pores having a diameter of between 70 and 130 Angstrom Units. Unfortunately, catalysts having a smaller pore size are usually more quickly deactivated by the deposition of metals within the pore structure than are catalysts having a larger pore size. Thus in selecting a suitable catalyst to remove contaminants from a heavy feedstock, it is necessary to balance catalyst life against the need to retain sufficient activity to remove the contaminants, especially carbon residue and sulfur.
In order to gain the advantages of both the lower activity catalysts for removing metals and of the higher activity catalysts needed for desulfurization and carbon residue removal, dual or multiple catalyst systems have been proposed for use in fixed bed reactors. Layered catalyst beds are proposed in U.S. Pat. Nos. 4,990,243 and 5,071,805 in which discrete strata of catalyst are arranged in the catalyst bed to take maximum advantage of the different characteristics of each of the catalysts making up the bed. In a layered catalyst bed the demetallation catalyst will usually make up the upper layer of the fixed bed with the catalyst in the lower layer or layers increasing in hydrogenation and desulfurization activity. The heavy feedstock enters the top of the reactor and first contacts the demetallation catalyst where the metal contaminants are removed. The heavy feedstock with a significant portion of its metal contaminants removed passes down through the fixed bed to contact the hydrogenation and desulfurization catalysts where the sulfur and carbon residue contaminants are removed. Due to the lowered metal values in the feedstock the hydrogenation and desulfurization catalysts will have an increased useful life since there are fewer metals present in the feedstock to deactivate the catalysts. However, a disadvantage of the downflow layered catalyst system is the high pressure drop which is typical across the fixed bed. This problem is further aggravated over time as the metals plate out on the catalyst in the upper layer of the bed increasing the pressure drop and eventually plugging the reactor.
The physical admixture of catalysts with differing activity in a fixed reactor bed is proposed in U.S. Pat. No. 5,439,860. This may have the advantage of more evenly distributing the metal contaminants throughout the length of the bed to reduce plugging, but it does not entirely solve the problem of deactivation of the hydrogenation catalysts by the metal contaminants. Much of the desired hydrogenation and desulfurization activity of the catalyst present in the upper portion of the fixed bed will be lost as the metal values plate out on the mixture of catalysts.
The use of a separate guard reactor containing primarily demetallation catalyst followed by a hydrotreating reactor containing the desulfurization catalysts is a known method of dealing with the problem of metals removal. See, for example, U.S. Pat. No. 5,779,992. This approach allows the relatively inexpensive demetallation catalyst in the guard reactor to be changed periodically as the metal contaminants build up in the guard reactor and increase the pressure drop. The more active and expensive hydrogenation catalysts in the hydrotreating reactor are protected from the metal contaminants and will last longer. However, such a system requires a significantly higher initial capital investment, since at least two reactors instead of one are necessary. In addition, in order to prevent the shutdown of the desulfurization reactor during catalyst changeout of the guard reactor, a swing guard reactor must be included in the scheme which results in an even greater upfront capital expenditure.
The use of a downward moving packed catalyst bed with an upflow reactor has been proposed in U.S. Pat. No. 5,076,908. This system has the advantage of being able to continuously add fresh catalyst to the top of the moving bed while the spent catalyst is withdrawn from the bottom of the reactor. In addition, since the feedstock enters the bed from the bottom of the reactor there is very low pressure drop across the catalyst bed as compared to the other systems, and the problem associated with plugging is virtually eliminated. The primary disadvantage of this system is the relatively high capital cost of the reactor and of the associated equipment needed for the addition and removal of catalyst.
Although fixed bed upflow reactors appear in the literature (see U.S. Pat. No. 5,522,983), the use of upflow fixed bed reactors for hydrotreating operations has not been practiced commercially because of the difficulty of designing and managing such a system. The closest practical experience to an upflow fixed bed reactor in a hydrotreating operation is that obtained using the moving bed reactor discussed above when the catalyst in the reactor is withdrawn intermittently as opposed to continuously. In this instance, the reactor is operated for a given period of time as if it were an upflow fixed bed reactor. However, the design of the moving bed reactor is not optimal for this type of operation and does not take full advantage of the fixed bed system. In addition, the moving bed reactor is not well designed for use with a layered catalyst bed system since the integrity of the different catalyst layers will be lost with the addition of fresh catalyst and the removal of spent catalyst unless additional ports are added to the reactor to add and withdraw catalyst from each of the catalyst layers. Since the capital cost of the high pressure ports in the reactor is high, the overall capital cost of such a system would be increased significantly. In addition, the operation of such a system would be complex and somewhat tedious when compared to other schemes.
The present invention combines the most advantageous features of each of the systems discussed above while minimizing the upfront capital expenditure for equipment. The present invention is particularly advantageous for retrofitting an existing reactor or for increasing the reactor volume where pressure drop is a constraint.
In its broadest aspect, the present invention is directed to a reactor system for treating a heavy feedstock which contains contaminants comprised of a least one metal and of sulfur and carbon residue to produce a product having a lowered amount of said contaminant or contaminants as compared to the heavy feedstock, the reactor system comprising (a) a vertical fixed catalyst bed having a top and bottom and at least a lower horizontal catalyst layer and an upper horizontal catalyst layer, wherein the catalyst in the lower horizontal catalyst layer is characterized as having lower hydrogenation activity than the catalyst in the upper horizontal catalyst layer; (b) fluid distributing means at the bottom of each catalyst layer for controlling and evenly distributing the flow of fluids through the catalyst layer; and (c) means for introducing a fluid comprising the heavy feedstock and hydrogen into the bottom of the fixed bed and withdrawing a product from the top of the fixed bed, whereby the fluid is introduced into the bottom of the fixed catalyst bed and flows generally upward through the fixed bed sequentially contacting first the catalyst in the lower horizontal catalyst layer and then contacting the catalyst in the upper horizontal catalyst layer at a sufficiently low flow rate so that the average expansion of the fixed bed does not exceed five percent.
Broadly, the present invention is also directed to a process for reducing metals, carbon residue, and sulfur contaminants in a heavy feedstock comprising the steps of (a) passing the heavy feedstock in the presence of hydrogen generally upwardly into a single reactor containing therein a vertical fixed bed having at least a lower horizontal catalyst layer and an upper horizontal catalyst layer, wherein the catalyst in the lower horizontal catalyst layer is characterized by having lower hydrogenation activity than the catalyst in the upper horizontal catalyst layer, (b) distributing the flow of heavy feedstock and hydrogen evenly across each catalyst layer as it passes upward, (c) maintaining a sufficiently low rate of flow for the heavy feedstock and hydrogen that the average expansion of the fixed bed does not exceed five percent, (d) sequentially contacting under hydrotreating conditions the heavy feedstock with the catalyst in the lower horizontal catalyst layer to remove a significant portion of the metals present followed by contacting the feedstock with the catalyst in the upper horizontal layer to remove additional metals and at least a portion of the sulfur and carbon residue, and (e) recovering a heavy feedstock product containing less metals and a lower content of carbon residue and sulfur as compared to the heavy feedstock. Preferably at least 50% of the metals will be removed from the heavy feedstock during the process.
The present invention is especially useful for hydrotreating a residuum, especially a metals containing residuum. A residuum for the purpose of this disclosure refers to a heavy feedstock that is high in asphaltenes and is collected from the bottoms of either an atmospheric distillation unit or a vacuum distillation unit. Atmospheric residuum has a boiling range above about 345 degrees C. (650 degrees F.). Vacuum residuum has a boiling range above about 540 degrees C. (1000 degrees F.) and usually will have a higher viscosity than atmospheric residuum. Vacuum residuum may require additional treatment to lower the viscosity of the feedstock. This may be accomplished by the addition of cutter, i.e. a lower viscosity material often recovered from the fractionator, to act as a diluent. It may also be desirable to carry out some visbreaking in the lower catalyst layer to reduce the viscosity of the feedstock.
As already noted, residuum is usually contaminated with various metals, as well as carbon residue and sulfur, which are preferably removed prior to further processing. In addition to the removal of the metals, carbon residue, and sulfur present in the heavy feedstock, the process and reactor system of the present invention will also remove nitrogen. The present invention is especially useful in the removal of metal contaminants from the feedstock, and is most advantageous when it is used to remove metal from a residuum. Typical metal contaminants that occur in residuum feedstocks include, but are not necessarily limited to, iron, vanadium, nickel, and calcium. The metals may be present as fairly simple compounds of the metals, such as oxides, metal halides, and such, or as more complex molecules, such as organometallic compounds. The contaminating metals are usually readily removed from the feedstock under the conditions present in the reactor of the present invention. Catalysts having low hydrogenation activity and relatively large pore diameters are usually preferred for demetallation.
Hydrogenation activity of a catalyst refers in this disclosure to the ability of the catalyst to remove carbon residue and heteroatoms, especially sulfur and nitrogen, from the hydrocarbon molecules in the feedstock in the presence of hydrogen. Thus low hydrogenation activity refers to a catalyst having relatively little ability to remove carbon residue, sulfur, or nitrogen as compared to a catalyst having higher hydrogenation activity which will more readily remove carbon residue, sulfur, and nitrogen.
Carbon residue, nitrogen, and sulfur are also commonly present as contaminants in the residuum. Although these contaminants may be present as relatively simple molecules, they are usually more tightly held in the hydrocarbon molecules by chemical bonds than the metals. Therefore, a somewhat more active hydrogenation catalyst is usually is required to remove the carbon residue, sulfur, and nitrogen than is necessary for the removal of metals. Catalysts suitable for the removal of carbon residue, nitrogen, and sulfur usually will also readily remove metals, but the catalysts become readily deactivated as the metals coat the surface of the catalyst and fill their pores.
Catalysts described in U.S. Pat. No. 5,472,928, the entire disclosure of which is herein incorporated by reference, are characterized by a narrow particle size distribution. Catalysts of this description are especially useful in the practice of the present invention and may be used to advantage in both the upper and lower catalyst layers of the reactor. Particularly preferred are spherical shaped catalysts having good crush strength, i.e., a crush strength of not less then 5 pounds. Crush strength is important to control the breakage of the catalyst particles.
An important aspect of the present invention is the upflow fixed bed design of the reactor. This differs from the typical downflow fixed bed designs which have been used to hydrotreat heavy feedstocks in the past. Due to the upward flow of fluid in the reactor, the upflow fixed bed differs from downflow fixed beds in that the upflow design has a lower pressure drop and a greater resistance to pressure drop buildup. In addition, since the present invention uses a fixed bed, i.e., one where there is relatively little movement of the catalyst particles, the present design is readily distinguished from ebullating bed designs. As already noted the flow of fluid upward through the catalyst bed is low enough to minimize the expansion of the catalyst bed as compared to the bed volume when no fluid is passing through the bed. The expansion of the catalyst bed should not exceed 5% and preferably will not exceed 2%. Ideally the expansion of the bed will be 0% or even a negative percentage, i.e., the volume of catalyst will decrease during operation. A second critical feature of the present invention is the layering of the catalyst to take maximum advantage of the characteristics of the catalysts making up each layer. The ebullating bed and moving bed designs cannot take advantage of a layered catalyst system since the movement of the catalyst particles will destroy the integrity of the individual layers. The present invention also differs from the moving bed design in that it does not require high pressure equipment for the addition and removal of catalyst. This results in a significant reduction in upfront capital costs. The fixed bed is also easier to manage and operate.
A further element of the invention is the fluid distributing means which is located at the bottom of each of the catalyst layers. The fluid distribution means may take a number of forms, such as, for example, screens, grids, perforated plates, and the like. The fluid distribution means serves two primary functions. It is intended to distribute the fluids passing upwardly through the reactor evenly across the horizontal plane of the catalyst layer. It also serves to insure the break-up of large gas bubbles and the optimal mixing of the fluids. Preferably, it also is designed to serve the secondary functions of supporting the catalyst layer and preventing the mixing of the catalyst particles at the interface between any two adjacent catalyst layers.
The present invention is intended for use with other hydroprocessing operations and is particularly valuable when used prior to one or more conventional hydrotreating units which are designed primarily for the removal of sulfur and carbon residue.